Holmes’ Tropisms of Animals. 
21 
that they are lost sight of in the more complex activities into 
which they enter as component factors. 
A most interesting field of investigation in this connection is 
presented in the relation of phototaxis and vision. It is a field 
scarcely touched upon as only one investigator, Radi, has entered 
upon it with any seriousness. There seems to be a close connec¬ 
tion in many animals, and especially insects, between phototaxis 
and what are called compensatory movements. Place a lady- 
beetle on a turn table which is slowly rotated. The beetle begins 
to move its head and then its body opposite the direction of move¬ 
ment. Robber flies show the reaction especially well. The reac¬ 
tion depends upon the eyes because it no longer occurs when the 
eyes are blackened over. It does not depend upon the rotation 
of the insect’s body. If the insect is placed in a cylinder on a sta¬ 
tionary center and the cylinder rotated, the insect tends to walk 
around in the direction of rotation. A frog under the same cir¬ 
cumstances will do the same thing. In these cases the animal 
reacts so as to keep, so far as possible, in statu quo with the visual 
field. 
A beautiful illustration of this is afforded by the so-called 
rheotropism of fishes. Many fishes have the instinct to head up 
stream against the current. This trait has been shown by Lyon to 
be dependent upon a visual reflex. He placed fish in an aquarium 
with the lower side made of glass below which could be drawn a 
long piece of cloth with alternate black and white stripes on it, 
giving the appearance of a moving bottom. As the strip was pulled 
along the fish swam in the direction of movement. Reversing the 
motion caused the fish to turn about and swim to the other end of 
the aquarium. In another experiment fishes were placed in a long 
bottle. When this floated down stream the fishes all swam to the 
up stream end. When it was pulled up stream the fishes all swam 
to the opposite end. Fishes in a stream, passively carried along, 
have no means of becoming aware of their movements except by 
means of objects in their field of vision any more than a man in a 
balloon who is carried along in a current of air. This automatic 
tendency to keep in constant relations with the objects in their 
field of vision keeps them from being passively carried down 
stream. Many insects show the same trait in their flying against 
